The panels of the fuselage of an aircraft generally comprise a skin that can be made of composite material as well as stiffeners in the form of sections that ensure the mechanical strength of the unit.
At the front and at the rear of the fuselage, the panels of the fuselage can have a double curvature, namely a first curvature that corresponds to the radius of the fuselage whose axis is parallel to the longitudinal axis of the aircraft and a second curvature whose axis is perpendicular to said longitudinal axis. Consequently, based on their location, the stiffeners can have a double curvature, namely a curved generatrix and a twisting for the section, or can exhibit localized joggling-type forms of unevenness, for example.
According to the embodiments, the stiffeners are connected to the skin by any known means such as riveting, gluing or co-baking.
To manufacture the stiffeners with a thermosetting resin matrix, one solution consists in producing draping from fabrics or layers of pre-impregnated fibers on a mandrel whose shape corresponds to that of the section to be obtained and then in baking the unit under vacuum so as to impart the desired mechanical properties to the stiffeners by polymerization of the resin.
According to this solution, the connection between the stiffener and the skin can be obtained by co-baking, whereby the two elements are flattened against one another during the baking.
This solution is not satisfactory because the draping operation on a three-dimensional shape is complex and is difficult to automate.
Furthermore, this solution does not make it possible to obtain high geometric precision, whereby only the surface that is in contact with the mandrel may be obtained with precise dimensions. Thus, this solution does not make it possible to obtain high precision with thicknesses and parallelism between the surface that is in contact with the mandrel and the opposite surface.
According to another problem, in the crude state, the fabric that is preimpregnated with a thermosetting resin has a period of use on the order of several days at ambient temperature. Also, the production flows should absolutely ensure the respect of the expiration period of all of the elements between the destocking time of the preimpregnated fabric and its implementation by draping and its baking. This constraint can prove problematic in the case of the production of a fuselage panel. Actually, in this case, it is necessary to produce all of the stiffeners, place them on a mandrel, drape the skin on the mandrel, and bake the unit before one of the elements has passed its expiration date.
Another solution consists in producing the sections by pultrusion. However, this technique does not make it possible to obtain stiffeners that are able to be added to a skin with a double curvature, exhibiting a rotation of sections around their axes of inertia. Furthermore, this technique makes it possible to obtain only certain orientations of the fibers. Also, the parts that are produced can be stressed only in a specific manner based on the orientation of said fibers.
In the case of parts that are produced from a thermoplastic resin matrix, it is possible to initiate the shaping of the stiffeners by forming. Such a technique is described in the document EP-1,543,942. In this case, the stiffeners are obtained from pre-consolidated blanks that are then formed after heating the latter above the melting point of the thermoplastic resin. The forming rests on the reversible transformation of the thermoplastic matrix, whereby the folds of fibers can glide relative to one another using the low viscosity of the matrix above its melting point.
This solution makes it possible to obtain high geometric precision and a level of productivity that is higher than those of other techniques.
However, the thermoplastic resins have certain drawbacks that reside within the cost of such resins and the fact that there is no industrial automated draping process that implements pre-impregnated materials that use this type of resin.
Furthermore, this technique cannot be transposed to thermosetting resin matrices. Actually, the latter hardens during polymerization in an irreversible manner, preventing any subsequent interlaminar slippage. Thus, in the case of a thermosetting resin, the interlaminar slippages can take place only in narrow slots of time and temperature, during which the polymerization of the resin has not reached a threshold beyond which said interlaminar slippages can no longer be produced. However, this time lapse is too short to make it possible to incorporate this stage in an industrial process.